Apply new scheduling group for multi-window mode apps

Currently it only applies for freeform window mode.
For example, if there are 4 visible freeform apps:
The top 2 will be SCHED_GROUP_TOP_APP.
And the bottom 2 will be SCHED_GROUP_FOREGROUND_MW.

That makes the non-top visible apps have higher priority than
SCHED_GROUP_DEFAULT, so they may be more responsive.

Bug: 200769420
Test: MockingOomAdjusterTests#testUpdateOomAdj_DoOne_VisibleActivities
Flag: com.android.window.flags.process_priority_policy_for_multi_window_mode
Change-Id: I2a78e3e4c7f67d2a1361810413486421e22f74b5